My assumption is that the "ideal" situation is steady shot fires .5 sec before auto shot is ready to fire. Also, Steady shot is fired as soon as each GCD is up. This would mean steady had a 1 sec cast and auto had a 1.5 sec cool down. This is neglecting player error and latency problems.
What I really want to know is what does and does not effect the cast time of steady shot. Does serpents swiftness? IAofT? Dragonspine trophy? Abbacus of Violent odds? Bloodlust? Passive item haste?
How do you know if it will effect steady shot or not?

Off topic but related, does haste affect Slam's castbar? How about Volley? Aimed Shot? Any other physical damage attacks with cast bars?

Aimed shot yes, volley no since it's a channeled affect and not a casting time. Not sure about slam but would be easy to check, pop abacus and cast slam.

Quiver haste does affect steady shot's cast time, every form of haste you can imagine will affect its cast time. There are numerous other threads where you could have gotten this information though.

Last patch they made haste effects work with channeled spells, i.e. Mind Flay and Drain Life. So it should work fine with volley.

From patch notes:
Spell haste now affects channeled spells, it says nothing about physical haste. Every other reply on this thread so far is correct.

The stickied theorycrafting post:
Working Theories of Theorycrafting as of 2.1
has the answer to your question in the first post.

It certainly doesn't have all the relevant information, some of that is in fact buried in 1000 post threads but it seems like a good place to start to look up information on any particular question.